Maleic anhydride had long been manufactured by the catalytic oxidation of benzene vapor with air. The gaseous effluent from such oxidation contains mainly nitrogen and unused oxygen together with oxides of carbon, water vapor, maleic anhydride vapor, and vapors of oxygen-containing organic compounds coproduced as impurities. Such gaseous effluent is cooled to a temperature below the boiling temperature of maleic anhydride but above 100.degree. C. to recover the anhydride as an impure liquid product. The resulting cooled gaseous mixture is scrubbed with a liquid to remove all of the maleic anhydride before venting the remaining gases and vapors. Maleic anhydride as second impure portion of product may be recovered from the scrubbing liquor and combined with the first portion of impure maleic anhydride product and then further processed at least by fractionation to recover purified maleic anhydride.
The quality specifications for commerially acceptable purified maleic anhydride include color values of molten anhydride both upon initial melting and after being held at 140.degree. C. for 90 minutes. The initial molten color should not exceed 20 and the aged color should not exceed 40 APHA Color Values as determined and described in ASTM test method D 3366-74.
To meet the aged molten color specification, the manufacturers of maleic anhydride found it necessary to subject the impure anhydride to a heat treatment step prior to fractionation so that colored body or color-forming impurities distilling with the anhydride would be converted to higher boiling substances. Such heat treatment was effectively maintained for periods of time which became economically and productively unattractive. The heat treatment of impure maleic anhydride was shortened by conducting this partial purification step in the presence of various addition agents. Following such time shortened heat treatment the mixture was subjected to a simple distillation to remove maleic anhydride and materials boiling at a temperature below the boiling temperature of said anhydride. The resulting partially purified maleic anhydride was then subjected to fractionation to remove the low boiling impurities as a top fraction, purified maleic anhydride product as an intermediate fraction, and leave whatever higher boiling impurities not before removed or which formed during distillation or fractionation as a bottom fraction.
Now the catalytic oxidation of butane vapor with air has become an important route for the manufacture of maleic anhydride. The gaseous effluent from the oxidation also contains mainly nitrogen and unused oxygen together with oxides of carbon, water vapor, maleic anhydride vapor, and vapors of oxygen-containing organic compounds co-produced as impurities. Although those oxygen-containing organic co-products which were colored bodies or color formers differed in character and nature from such colored body and color formers in impure maleic anhydride product from the vapor phase benzene oxidation, similar heat treatment steps were successfully applied to assist in the removal of the impurities prior to or during the final fractionation purification.
Sulfuric acid, sulfur trioxide, oleum or sodium acid sulfate are taught in U.S. Pat. No. 2,296,218 as being useful as time shortening addition agents for the heat treatment partial purification of impure maleic anhydride obtained from benzene vapor oxidation. However, we found that sulfuric acid at the 0.5 weight percent concentration provided a corrosive environment as a time shortening addition agent for the partial purification of maleic anhydride obtained from butane vapor oxidation with air.
We have now found a co-additive or adjunct additive for use with sulfuric acid during the heat treatment partial purification step used prior to final fractionation which provides the sought improvement of aged molten color of maleic abhydride obtained from air oxidation of butane vapor.